Django -在现有QuerySet前面添加对象

Question

我正在构建一个library Django应用程序，其中我有一个BooksView来显示由某个Author编写的所有Book。在某些情况下，我希望在查询集中的所有其他书籍之前显示单个Book。当图书的id出现在URL中时，就会发生这种情况，否则，所有的书籍都应该按时间顺序列出。

图书/urls.py

urlpatterns = [

    # Books displayed in chronological order
    path("<slug:author>/books", views.BooksView.as_view()),

    # Books displayed in chronological order
    # but first book is the one with id `book`
    path("<slug:author>/books/<int:book>", views.BooksView.as_view()),
]

books/views.py

class BooksView(APIView):

    def get(self, request, author, book=None):

        author = get_object_or_404(Author, slug=author)
        books = author.author_books
        books = books.order_by("-pub_date")

        if book:
            book = get_object_or_404(Book, id=book)
            # TODO: add `book` in front of books QuerySet (and remove duplicated if any)

        serializer = BooksSerializer(books, many=True)
        return Response(serializer.data)

我如何完成我的TODO？如何将对象推送到现有查询集的前面？

Answer 1

这并不能准确地回答您的问题，但我认为它将允许您获得预期的结果:不要担心保存查询集，而是将其转换为列表。

对于整个代码库中的每个查询集，这都不是您想要做的事情，特别是如果它是您以后可能想要构建并与其他过滤器、排除等一起构建和组合的查询集。但是，当您要呈现一个页面(就像您在这里)时，它是可以的，因为无论如何您都要对查询集进行评估。你会让它更快发生--也许是微秒。

class BooksView(APIView):

    def get(self, request, author, book=None):

        author = get_object_or_404(Author, slug=author)
        books = list(author.author_books)

        if book:
            book = Book.objects.get(id=book)
            # TODO: add `book` in front of books QuerySet (and remove duplicated if any)
            books = [x for x in books if not x == book] # first remove if dup
            books.insert(0, book) # now insert at front

        serializer = BooksSerializer(books, many=True)
        return Response(serializer.data)

编辑1

BooksSerializer (我怀疑它是BaseSerializer的子类)将在您调用它时立即列出一个列表：

    def to_representation(self, data):
        """
        List of object instances -> List of dicts of primitive datatypes.
        """
        # Dealing with nested relationships, data can be a Manager,
        # so, first get a queryset from the Manager if needed
        iterable = data.all() if isinstance(data, models.Manager) else data

        return [
            self.child.to_representation(item) for item in iterable
        ]

框架/序列化器.#L 663

编辑2

不如试试这个吧？通过在计算查询集之前将exclude添加到列表中，可以防止O(n)扫描到列表中，以查找和删除应该位于顶部的“主”书。

class BooksView(APIView):

    def get(self, request, author, book=None):

        author = get_object_or_404(Author, slug=author)
        books = author.author_books

        if book:
            book = Book.objects.get(id=book)
            # TODO: add `book` in front of books QuerySet (and remove duplicated if any)
            # ensure the queryset doesn't include the "main" book
            books = books.exclude(book_id=book.id)
            # evaluate it into a list just like the BookSerializer will anyhow
            books = list(books)
            # now insert the "main" book at the front of the list 
            books.insert(0, book)

        serializer = BooksSerializer(books, many=True)
        return Response(serializer.data)

Answer 2

这应该可以做到，只需在QuerySet对象中使用union方法，并将我们试图从books查询集中访问的书籍排除在外。

我对代码做了一些小的修改，但是您不需要做更多的事情来完成您需要的事情。

class BooksView(APIView):

    def get(self, request, author, book=None):
        author = get_object_or_404(Author, slug=author)
        books = author.author_books.all().order_by('-pub_date')

        if book:
            # I am assuming this line if for validating that the request is valid.
            # I've changed the query to also include author's slug,
            # so the request doesn't get books not related to the author.
            book_obj = get_object_or_404(Book, id=book, author__slug=author)

            # Just Query the same book, and union it with the books queryset,
            # excluding the current book in the book_obj
            books = Book.objects.filter(id=book_obj.id).union(books.exclude(id=book_obj.id))

        serializer = BooksSerializer(books, many=True)
        return Response(serializer.data)

Answer 3

也许有点出格了，但是如果你想把它保持为queryset，那么这就是解决方案。请注意，这解决了X问题，而不是Y问题。其目标是将一本书放在列表的第一位，这可以通过插入实现，也可以通过基于注释的重新排序来实现。

from django.db.models import Case, When, Value, IntegerField
from rest_framework import generics

from . import models, serializers


class BooksPerAuthor(generics.ListAPIView):
    serializer_class = serializers.BookWithoutAuthor

    def get_queryset(self):
        book_pk = self.kwargs.get("book", 0)
        queryset = (
            models.Book.objects.filter(author__slug=self.kwargs["author"])
            .annotate(
                promoted=Case(
                    When(pk=book_pk, then=Value(1)),
                    default=Value(0),
                    output_field=IntegerField(),
                )
            )
            .order_by("-promoted", "-pub_date")
        )
        return queryset

这不会捕捉到不存在的引用--不喜欢返回404作为列表视图，而且如果推广的书籍不在集合中，那么就没有真正的危害了。上面的许多例子都是以额外查询的(较小的)代价来处理404的。